1. Field of the Invention
The present invention relates to modulated optical sources and particularly to an apparatus for generating a high-power, modulated optical output.
2. Description of the Prior Art
The current state of development of diffraction-limited laser diodes and diffraction-limited diode amplifiers can be grouped into five categories.
A first category pertains to single, narrow-stripe laser diodes operating in the fundamental spatial mode at power levels up to 200 milliwatts (mW). Lasers in this category can probably be directly modulated out to one or two gigabits per second (Gbits/s), but that is the limit due to electrical parasitics. Furthermore, the output power level in discrete devices probably will not go much higher than a few hundred milliwatts, which is the reason why laser diode arrays and broad stripe lasers have been developed.
A second category includes coupled stripe laser arrays and broad stripe lasers emitting in a diffraction-limited, far field lobe, under free running operation. In this category, truly diffraction-limited outputs have been difficult to achieve. In some structures a near diffraction-limited beam can be obtained only over a narrow range of laser diode current. Also, in most cases, even where there is a diffraction-limited lobe, most of the energy lies outside of the lobe and, thus, only a small amount can be used with diffraction-limited coupling optics.
A third category involves the injection-locking of a coupled stripe laser array/broad stripe laser using an external master laser. This third category of injection-locking has served to achieve diffraction limited emission of the laser diode array or broad stripe laser, making most of the far field usable for focussing or collimation, as well as improving the spectral output of the slave laser diode array or broad stripe laser. Frequency modulation has been accomplished by the simultaneous current modulation of the master laser and slave laser diode array (or slave broad stripe laser) to keep the two lasers within the locking bandwidth. That technique is limited to low bandwidth frequency modulation and does not lend itself to data transmission except in an extremely simple format.
A fourth category relates to coupled stripe laser diode arrays and broad stripe lasers operating in external cavities. This fourth category of using external cavities suffers from a limited modulation bandwidth due to a long photon lifetime inside the cavity which is normally centimeters in length.
A fifth category comprises single pass or resonant high-power, large active area diode amplifier with an input optical signal from a low-power master laser.
An alternative approach that could be used is an external modulator which phase or intensity modulates an optical carrier and a laser diode array/broad area laser or amplifier which effectively amplifies the output power of the modulator. Bulk modulators normally require high electric fields and have transit time limitations. These disadvantages led to the development of integrated optic modulators, normally in lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3). The main problem in these modulators is optical damage or photorefractive changes due to the high fields in the two-dimensional waveguides fabricated in these materials.
Recently there have been several new developments in waveguide fabrication where optical damage can be avoided. A first new development involves waveguides fabricated in potassium titanyl phosphate (KTiOPO.sub.4 or KTP), as reported by Bierlein et al. ("Fabrication and Characterization of Optical Waveguides in KTP", paper PDP5, OFC, Reno, 1987). Another waveguide system that uses proton exchange in lithium tantalate also has a substantially higher optical damage threshold than LiNbO.sub.3. No optical damage was observed at a 4 mW power output from a single mode waveguide of such a waveguide system at a wavelength of 0.633 microns. (See T. Findalky et al., "High Quality LiTaO.sub.3 Integrated-Optical Waveguides and Devices Fabricated by the Annealed Proton-Exchange Technique", Opt. Letts., Vol. 13, 797, 1988.)
Heretofore, no high-power, modulated optical source has utilized these newly developed, damage-resistant waveguides with injection-locked arrays, semiconductor amplifiers or external cavity lasers to obtain a phase- or intensity-modulated, high-power optical source that is compact with a bandwidth in the multi-gigahertz range.